Conventionally, a portable terminal, such as a portable telephone and a portable information terminal, using a primary cell or secondary cell as a sub-cell to back up memory is commercially available.
This sub-cell is, when a main power source as a driving power source in normal operation is removed to charge or cell-exchange, used 1) to conduct a suspend-resume operation to allow the operation state to return to a state just before turning off the main power source when turning on the main power source next time, 2) to retain information stored in memory, or 3) to retain information stored in memory even after the exhaustion of capacity in the main power source.
Further, in case of a device with built-in calendar, an internal circuit to count the calendar time is backed up by the sub-cell for backing up memory to keep on counting the time even when a normal current is not supplied from the main power source.
Also, as a method for restoring contents of main memory being lost when turning off the main power source, there is a method that necessary information, such as data, application program and system program, e.g., OS, is stored into a file in non-erasable area, then loaded from the file to memory when starting the operation. For example, Japanese patent application laid-open No. 4-098473 (1992) discloses a method of loading from non-volatile memory.
However, even in such a method of loading from non-volatile memory, the information needs to be backed up while being loaded on the memory in order to conduct the suspend-resume operation mentioned above before and after exchanging the main cell. Namely, it also needs a sub-cell as the portable terminal does.
However, there is conventionally a problem that both memory and calendar are lost when the sub-cell is exhausted because one sub-cell is used for backing up both the memory and calendar. Furthermore, after reviving the main power source, through, as to memory, necessary information can be loaded by using, e.g., the method of loading from non-volatile memory when turnings on the power source next time, calendar is necessary to re-input, which takes time.
Also, in case of a device that employs the method of loading from non-volatile memory, as shown in Table 1, there is a problem that a same sub-cell is used to back up memory as to which relatively large back-up current is necessary for, e.g., the suspend-resume operation during the exchange of main cell but the back-up time is allowed to be short, and calendar as to which back-up current is allowed to small but it is desired to retain for a long period.
TABLE 1 ______________________________________ Desired Required Back-up Object Current Period Restoring Method ______________________________________ memory large short time loading from (max. 24 hrs.) non-volatile memory calendar small long time re-inputting date (longer than 5 yrs.) and time manually ______________________________________